In recent years, the demand for secondary batteries has increased as power sources for transportation systems such as electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles and for large-scale electricity storage devices for home and commercial use. As these power sources, lithium secondary batteries are widely used. In lithium secondary batteries, lithium ions are used as carrier ions. However, lithium is a rare metal. It is a limited natural resource and expensive.
As an alternative to lithium secondary batteries, sodium secondary batteries have been studied. In sodium secondary batteries, sodium ions are used as carrier ions. Since sodium is more abundant and less expensive than lithium, sodium secondary batteries have attracted attention as inexpensive secondary batteries applicable to large-scale systems. However, sodium secondary batteries are still under research and development, and their electricity storage capacities are lower than those of lithium secondary batteries. Therefore, practical use of sodium secondary batteries requires development of higher capacity materials for positive and negative electrodes.
The use of a Na-containing transition metal oxide as a positive electrode material for sodium secondary batteries is reported. Patent Literature 1 discloses a composite metal oxide containing Na, Mn, and M1 (where M1 is Fe or Ni), with a Na:Mn:M1 molar ratio being a:1-b:b (where a is a value more than 0.5 and less than 1, and b is a value of 0.001 or more and 0.5 or less). Specifically, Patent Literature 1 discloses material compositions such as Na0.7Mn0.75Fe0.25O2 and Na0.7Mn0.75Ni0.25O2. According to this disclosure, when Na0.7Mn0.75Fe0.25O2 and
Na0.7Mn0.75Ni0.25O2 are used as positive electrode active materials for sodium secondary batteries, the resulting charge-discharge capacities are 120 mAh/g and 119 mAh/g, respectively, after 10 cycles.
Patent Literature 2 discloses a mixture material containing a composite metal oxide composed of an oxide represented by the formula NaxFeyMn1−yO2 (where x is a value of more than ⅔ and less than 1, and y is a value of more than 0 and less than ⅔) and having a P2 structure and a layered oxide having an octahedral structure and/or a triangular-prism structure as a stacking fault. Specifically, Patent Literature 2 discloses a mixture material having a composition of Na0.73Mn0.45Fe0.55O2, composed of an oxide having the P2 structure and an oxide having the O3 structure and P3 structure. According to this disclosure, when this mixture material is used as a positive electrode active material for sodium secondary batteries, the resulting capacity is at least about 150 mAh/g.
Non-patent Literature 1 reports that when Na2/3[Fe1/2Mn1/2]O2 having the P2 structure is used as a positive electrode of a sodium secondary battery, the highest capacity of about 190 mAh/g can be obtained. It is also reported that when as another positive electrode material having a similar composition but a different crystal structure, Na[Fe1/2Mn1/2]O2 having the O3 structure is used as a positive electrode of a sodium secondary battery, the resulting capacity is only about 100 to 110 mAh/g, which means that not only the composition but also the crystal structure is important. Non-patent Literature 1 also discloses Na0.6MnO2, Na0.7CoO2, and Na2/3[Ni1/3Mn2/3]O2 as other Na-containing transition metal oxides having the P2 structure and showing high capacities as described above.
In addition, for example, Patent Literature 2 discloses an example of the method for producing these Na-containing transition metal oxides, in which a Na compound and a transition metal oxide are mixed at a predetermined ratio and the resulting mixture is calcined in air at 900° C. for 12 hours to obtain a Na-containing transition metal oxide.